The improved understanding of various physiological regulatory pathways facilitated through the research efforts in genomics and proteomics has begun to impact the discovery of novel pharmaceutical agents. In particular, the identification of key receptors and their endogenous ligands has created new opportunities for exploitation of these receptor/ligand pairs as therapeutic targets. For example, ghrelin is a recently characterized 28-amino acid peptide hormone isolated originally from the stomach of rats with the orthologue subsequently identified in humans. (Kojima, M.; Hosoda, H. et al. Nature 1999, 402, 656-660.) The existence of this peptide in a range of other species suggests a conserved and important role in normal body function. This peptide has been demonstrated to be the endogenous ligand for a previously orphan G protein-coupled receptor (GPCR), type 1 growth hormone secretatogue receptor (hGHS-R1a) (Howard, A. D.; Feighner, S. D.; et al. Science 1996, 273, 974-977) found predominantly in the brain (arcuate nucleus and ventromedial nucleus in the hypothalamus, hippocampus and substantia nigra) and pituitary. (U.S. Pat. No. 6,242,199; Intl. Pat. Appl. Nos. WO 97/21730 and WO 97/22004) hGHS-R1a has recently been reclassified as the ghrelin receptor (GHRN) in recognition of its endogenous ligand (Davenport, A. P.; et al. Pharmacol. Rev. 2005, 57, 541-546). The receptor has also been detected in other areas of the central nervous system (CNS) and in peripheral tissues, for instance adrenal and thyroid glands, heart, lung, kidney, and skeletal muscle. This receptor was identified and cloned prior to the isolation and characterization of the endogenous peptide ligand and is distinct from other receptors involved in the regulation of growth hormone (GH) secretion, in particular, the growth hormone-releasing hormone (GHRH) receptor.
A unique characteristic of both the rat and human peptides is the presence of the n-octanoyl (Oct) moiety on Ser3. However, the des-acyl form predominates in circulation, with approximately 90% of the hormone in this form. This group is derived from a post-translational modification and appears relevant for bioactivity and possibly also for transport into the CNS. (Banks, W. A.; Tschop, M.; Robinson, S. M.; Heiman, M. L. J. Pharmacol. Exp. Ther. 2002, 302, 822-827.) In a GH-releasing assay, the des-octanoyl form of the hormone was at least 100-fold less potent than the parent peptide, although it has been suggested that the des-acyl species may be responsible for some of the other biological effects associated with ghrelin. This des-acyl form has also been postulated to be primarily responsible for the cardiovascular and cell proliferation effects attributed to ghrelin, while the acylated form participates in maintenance of energy balance and growth hormone release. (Baldanzi, G.; Filighenddu, N.; Cutrupi, S.; et al. J. Cell Biol. 2002, 159, 1029-1037.) Similarly, des-Gln14-ghrelin and its octanoylated derivative have been isolated as endogenous forms of the hormone arising from alternative splicing of the ghrelin gene, but both are found to be inactive in stimulating GH release in vivo. (Hosoda, H.; Kojima, M.; Matsuo, H.; Kangawa, K. J. Biol. Chem. 2000, 275, 21995-2120.) Other minor forms of ghrelin produced by post-translational processing have been observed in plasma, although no specific activity has been attributed to them. (Hosoda, H.; Kojima, M.; et al. J. Biol. Chem. 2003, 278, 64-70.)
Even prior to the isolation of this receptor and its endogenous peptide ligand, a significant amount of research was devoted to finding agents that can stimulate GH secretion. The proper regulation of human GH has significance not only for proper body growth, but also a range of other critical physiological effects. Since GH and other GH-stimulating peptides, such as GHRH and growth hormone releasing factor (GRF), as well as their derivatives and analogues, are administered via injection, to better take advantage of these positive effects, attention was focused on the development of orally active therapeutic agents that would increase GH secretion, termed GH secretagogues (GHS). Additionally, use of these oral agents was expected to more closely mimic the pulsatile physiological release of GH.
Beginning with the identification of the growth hormone-releasing peptides (GHRP) in the late 1970's (Bowers, C. Y. Curr. Opin. Endocrinol. Diabetes 2000, 7, 168-174; Camanni, F.; Ghigo, E.; Arvat, E. Front. Neurosci. 1998, 19, 47-72; Locatelli, V.; Torsello, A. Pharmacol. Res. 1997, 36, 415-423), a host of agents have been studied for their potential to act as GHS. In addition to their stimulation of GH release and concomitant positive effects in that regard, GHS were projected to have utility in the treatment of a variety of other disorders, including wasting conditions (cachexia) as seen in HIV/AIDS patients and cancer-induced anorexia, musculoskeletal frailty in the elderly, and growth hormone deficient diseases. Many efforts over the past 25 years have yielded a number of potent, orally available GHS. (Rocha-Sousa, A.; Henriques-Coelho, T.; Leite_Moreira, A. F. Exp. Opin. Ther. Patents 2007, 17, 909-926; Isidro, M. L.; Cordido, F. Comb. Chem. High Throughput Screen. 2006, 9, 178-180; Smith, R. G.; Sun, Y. X.; Beatancourt, L.; Asnicar, M. Best Pract. Res. Clin. Endocrinol. Metab. 2004, 18, 333-347; Fehrentz, J.-A.; Martinez, J.; Boeglin, D.; Guerlavais, V.; Deghenghi, R. IDrugs 2002, 5, 804-814; Svensson, J. Exp. Opin. Ther. Patents 2000, 10, 1071-1080; Nargund, R. P.; Patchett, A. A.; et al. J. Med. Chem. 1998, 41, 3103-3127; Ghigo, E; Arvat, E.; Camanni, F. Ann. Med. 1998, 30, 159-168; Smith, R. G.; Van der Ploeg, L. H. T.; Howard, A. D.; Feighner, S. D.; et al. Endocr. Rev. 1997, 18, 621-645.) These include small peptides, such as hexarelin (Zentaris) and ipamorelin (Novo Nordisk), and adenosine analogues, as well as small molecules such as capromorelin (Pfizer), L-252,564 (Merck), MK-0677 (Merck), NN703 (tabimorelin, Novo Nordisk), G-7203 (Genentech), S-37435 (Kaken) and SM-130868 (Sumitomo), BMS-604992 (Bristol-Myers Squibb) and RC-1291 (anamorelin, Sapphire) designed to be orally active for the stimulation of growth hormone. However, clinical testing with such agents have rendered disappointing results due to, among other things, lack of efficacy over prolonged treatment or undesired side effects, including irreversible inhibition of cytochrome P450 enzymes (Zdravkovic M.; Olse, A. K.; Christiansen, T.; et al. Eur. J. Clin. Pharmacol. 2003, 58, 683-688.) Therefore, there remains a need for pharmacological agents that could effectively target the ghrelin receptor for therapeutic action.
Despite its involvement in GH modulation, ghrelin is primarily synthesized in the oxyntic gland of the stomach, although it is also produced in lesser amounts in other organs, including the kidney, pancreas and hypothalamus. (Kojima, M.; Hsoda, H.; Kangawa, K. Horm. Res. 2001, 56 (Suppl. 1), 93-97; Ariyasu, H.; Takaya, K.; Tagami, T.; et al. Stomach is a major source of circulating ghrelin, and feeding state determines plasma ghrelin-like immunoreactivity levels in humans. J. Clin. Endocrinol. Metab. 2001, 86, 4753-4758.) In addition to its role in stimulating GH release, the hormone has a variety of other endocrine and non-endocrine functions (Broglio, F.; Gottero, C.; Arvat, E.; Ghigo, E. Horm. Res. 2003, 59, 109-117) and has been shown to interact with a number of other physiological systems in maintaining proper energy balance. (Horvath, T. L.; Diano, S.; Sotonyi, P.; Heiman, M.; Tschöp, M. Endocrinology 2001, 142, 4163-4169; Casanueva, F. F.; Dieguez, C. Rev. Endocrinol. Metab. Disord. 2002, 3, 325-338). In particular, ghrelin plays a role as an orexigenic signal in the control of feeding, in which it acts to counteract the effects of leptin. Indeed, it was the first gut peptide proven to have such orexigenic properties. (Kojima, M.; Kangawa, K. Curr. Opin. Pharmacology 2002, 2, 665-668.) The hormone also is implicated in the hypothalamic regulation of the synthesis and secretion of a number of other neuropeptides involved in appetite and feeding behavior. Levels of ghrelin are elevated in response to fasting or extended food restriction. (Nakazato, M.; Murakami, N.; Date, Y.; Kojima, M.; et al. Nature 2001, 409, 194-198.) For example, subjects suffering with anorexia or bulimia exhibit elevated ghrelin levels. Circulating levels of the hormone have been found to rise before meals and fall after meals. In addition, diet-induced weight loss leads to increased ghrelin levels, although obese subjects who have gastric bypass surgery do not likewise experience such an increase. (Cummings, D. E.; Weigle, D. S.; Frayo, R. S.; et al. N. Engl. J. Med. 2002, 346, 1623-1630)
This intimate involvement of ghrelin in control of food intake and appetite has made it an attractive target for obesity research. (Spanswick, D.; Lee, K. Exp. Opin. Emerging Drugs 2003, 8, 217-237; Horvath, T. L.; Castañeda, T.; Tang-Christensen, M.; Pagotto, U.; Tschöp, M. H. Curr. Pharm. Design 2003, 9, 1383-1395; Crowley, V. E. F.; Yeo, G. S. H.; O-Rahilly, S. Nat. Rev. Drug Disc. 2002, 1, 276-286.) Indeed, few other natural substances have been demonstrated to be involved in the modulation of both GH secretion and food intake.
Similarly, ghrelin plays a role in the regulation of insulin release and glycemia and hence modulators of the ghrelin receptor have application to the treatment of diabetes and metabolic syndrome. (Yada, T.; Dezaki, K. Sone, H.; et al. Curr. Diab. Rev. 2008, 4, 18-23).
Also, as previously mentioned with respect to the GHS, ghrelin and ghrelin agonists have been demonstrated to have positive effects in wasting syndromes and cachexia. Clinical trials have been initiated to take advantage of these effects. (Strasser, F.; Lutz, T. A.; Maeder, M. T. Br. J. Cancer 2008, 98, 300-308; Garcia, J. M.; Polyino, W. J. The Oncologist 2007, 12, 594-600.)
An additional effect of ghrelin that has not been exploited to date for therapeutic purposes is in modulating gastric motility and gastric acid secretion. The pro-kinetic activity appears to be independent of the GH-secretory action and is likely mediated by the vagal-cholinergic muscarinic pathway. The dose levels required are equivalent to those necessary for the hormone's GH and appetite stimulation actions. It is noteworthy that, in contrast to its inactivity for ghrelin's other actions, the des-Gln14 peptide demonstrated promotion of motility as well. (Chen, C.-Y.; Inui, A.; Asakawa, A.; Fujino, K.; Kato, I.; Chen, C.-C.; Ueno, N.; Fujimiya, M. Gastroenterology 2005, 129, 8-25; Chen, C.-Y.; Chao, Y.; Chang, F.-Y.; Chien, E. J.; Lee, S.-D.; Doong, M.-L. Int. J. Mol. Med. 2005, 16, 695-699; Trudel, L.; Bouin, M.; Tomasetto, C.; Eberling, P.; St-Pierre, S.; Bannon, P.; L'Heureux, M. C.; Poitras, P. Peptides 2003, 24, 531-534; Trudel, L.; Tomasetto, C.; Rio, M. C.; Bouin, M.; Plourde, V.; Eberling, P.; Poitras, P. Am. J. Physiol. 2002, 282, G948-G952; Peeters, T. L. J. Physiol. Pharmacol. 2003, 54(Supp. 4), 95-103.)
A growing amount of evidence has demonstrated ghrelin to be a regulator of inflammation and immune function. (Taub, D. D. Vitamins and Hormones 2007, 77, 325-346; Vixit, V. D.; Taub, D. D. Exp. Gerontol. 2005, 40, 900-910.) Ghrelin specifically inhibits the expression of pro-inflammatory cytokines such as IL-1β, IL-6 and TNF-α in human monocytes and T cells (Dixit, V. D.; Schaffer, E. M.; et al. J. Clin. Invest. 2004, 114, 57-66). Ghrelin exhibits novel anti-inflammatory actions in human endothelial cells through deactivation of the NF-κB pathway. (Li, W. G.; Gavrila, D.; Liu, X.; et al. Circulation 2004, 109, 2221-2226; Zhao, D.; Zhan, Y.; Zeng, H.; et al. J. Cell. Biochem. 2006, 97, 1317-1327.) Ghrelin exerts a protective effect on the gastric mucosa mediated in part through prostaglandins. (Konturek, P. C.; Brzozowski, T.; Pajdo, R.; et al. J. Physiol. Pharmacol. 2004, 55, 325-336.) Ghrelin levels are elevated in patients with IBD (Peracchi, M.; Bardella, M. T.; et al. Gut 2006, 55, 432-433; Karmiris, K.; Koutroubakis, I. E.; et al. Inflamm. Bowel Dis. 2006, 12, 100-105), colitis (Gonzalez-Rey, E.; Chorny, A.; Delgado, M. Gastroenterology 2006, 130, 1707-1720), peptic ulcer disease (Suzuki, H.; Masaoka, T.; Nomoto, Y.; et al. Alignment. Pharmacol. Ther. Symp. Ser. 2006, 2, 120-126), duodenal ulcers (Fukuhara, S.; Suzuki, H.; Masaoka, T.; et al. Am. J. Physiol. 2004, 289, G138-G145) and postoperative intra-abdominal sepsis (Maruna, P.; Gürlich, R.; Frasko, R.; Rosicka, M. Eur. Surg. Res. 2005, 37, 354-359), but decreased in rheumatoid arthritis (Otero, M.; Nogueiras, R.; et al. Rheumatol. 2004, 43, 306-310). In rat models, ghrelin peptide protects against or improves ischemia-reperfusion injury (Konturek, P. C.; Brzozowski, T.; et al. Eur. J. Pharmacol. 2006, 536, 171-181), pancreatic and liver damage (Kasimay, O.; Iseri, S. O.; Barlas, A.; et al. Hepatol. Res. 2006, 36, 11-19), acute pancreatitis (Dembinski, A.; Warzecha, Z.; et al. J. Physiol. Pharmacol. 2003, 54, 561-573), sepsis and septic shock (Wu, R.; Dong, W.; Cui, X.; et al. Ann. Surg. 2007, 245, 480-486; Chang, L.; Lu, J.-B.; et al. Acta Pharmacol. Sin. 2003, 24, 45-49), gastric damage caused by certain drugs (Iseri, S.; Sener, G.; et al. J. Endocrinol. 2005, 187, 399-406), stress-induced gastric damage (Brzozowski, T.; Konturek, P. C.; Konturek, S. J.; et al. Regul. Pept. 2004, 120, 39-51), gastric damage caused by H. pylori (Isomoto, H.; Ueno, H.; et al. Dig. Dis. Sci. 2005, 50, 833-838) and inflammatory pain (Sibilia, V.; Lattuada, N.; et al. Neuropharmacology 2006, 51, 497-505). Ghrelin is, as well, associated with chronic kidney disease (Stenvinkel, P.; Pecoits-Filho, R.; Lindholm, B. Adv. Renal Replacement Ther. 2003, 10, 332-3450). Further, peptide agonists have proven efficacious in animal models, including GHRP-2 for arthritis in the rat (Granado, M.; Priego, T.; et al. Am. J. Physiol. 2005, 288, E486-E492; Am. J. Physiol. 2005, 289, E1007) and GHRP-6 for acute ischemia in dogs (Shen, Y.-T.; Lynch, J. J.; Hargreaves, R. J.; Gould, R. J. J. Pharmacol. Exp. Ther. 2003, 306, 815-820.) Ghrelin and ghrelin agonists hence can be applied to the treatment and prevention of inflammatory disorders. Interestingly, ghrelin antagonists have been described to be useful for the treatment of intestinal inflammation (U.S. Pat. Appl. Publ. 2007/0025991).
Ghrelin also has been implicated in various aspects of reproduction and neonatal development. (Arvat, E.; Gianotti, L.; Giordano, R.; et al. Endocrine 2001, 14, 35-43.) Also of significance are the cardiovascular effects of ghrelin, since the peptide is a powerful vasodilator. As such, ghrelin agonists have potential for the treatment of chronic heart failure. (Nagaya, N.; Kangawa, K. Regul. Pept. 2003, 114, 71-77; Nagaya, N.; Kangawa, K. Curr. Opin. Pharmacol. 2003, 3, 146-151; Bedendi, I.; Alloatti, G.; Marcantoni, A.; Malan, D.; Catapano, F.; Ghé, C.; et al. Eur. J. Pharmacol. 2003, 476, 87-95; Isgaard, J.; Johansson, I. J. Endocrinol. Invest. 2005, 28, 838-842.) Intl. Pat. Appl. Publ. WO 2004/014412 describes the use of ghrelin agonists for the protection of cell death in myocardial cells and as a cardioprotectant treatment for conditions leading to heart failure.
Ghrelin has also been implicated in the regulation of bone metabolism. (van der Velde, M.; Delhanty, P.; et al. Vitamins and Hormones 2008, 77, 239-258). Ghrelin and its receptor, GHS-R1a, were identified in osteoblasts, and ghrelin promoted both proliferation and differentiation. Furthermore, ghrelin increased bone mineral density and directly affects bone formation in rats. (Fukushima, N.; Hanada, R.; Teranishi, H.; et al. J. Bone Mineral Res. 2005, 20, 790-798).
Additionally, ghrelin peptide has been demonstrated to possess potent inhibitory effects on angiogenesis in vitro and in vivo. (Baiguera, S.; Conconi, M. T.; Guidolin, D.; et al. Int. J. Mol. Med. 2004, 14, 849-854; Conconi, M. T.; Nico, B.; Guidolin, D.; et al. Peptides 2004, 25, 2179-2185.)
Further, evidence also has been obtained that ghrelin may have implications in anxiety and other CNS disorders as well as the improvement of memory. (Carlini, V. P., Monzon, M. E., Varas, M. M., Cragnolini, A. B., Schioth, H. B., Scimonelli, T. N., de Barioglio, S. R. Biochem. Biophys. Res. Commun. 2002, 299, 739-743; Diano, S.; Farr, S. A.; Benoit, S. C.; et al. Nature Neurosci. 2006, 9, 381-388; McNay, E. C. Curr. Opin. Pharmacol. 2007, 7, 628-632.) Lastly, glirelin has also been demonstrated to have effects on the regulation of sleep. (Szentirmai, E.; Kapás, L.; Krueger, J. M. Am. J. Physiol. Regul. Integr. Comp. Physiol. 2007, 292, R575-R585; Szentirmai, E.; Hajdu, I.; Obal, Jr, F.; Krueger, J. M. Brain Res. 2006, 1088, 131-140; Yannielli, P. C.; Molyneux, P. C.; Harrington, M. E.; Golombek, D. A. J. Neurosci. 2007, 2890-2895; Tolle, V.; Bassant, M.-H.; Zizzari, P. Endocrinology 2002, 143, 1353-1361.) However, the sleep-wake cycle in ghrelin knock-out mice has been reported to be normal, indicating that the regulatory situation might be more complex. (Szentirmai, E.; Kapás, L.; et al. Am. J. Physiol. Regul. Integr. Comp. Physiol. 2007, 293, R510-R517.) Ghrelin agonists have utility therefore as treatments for preventing or ameliorating conditions involving the CNS, including anxiety, stress, cognitive enhancement, and sleep regulation.
WO 2005/097174 and WO 2006/045314 discuss the use of GHS, ghrelin and other peptides or combinations thereof for the treatment of cachexia and chronic obstructive pulmonary disease, respectively. WO 2005/09726 reports on GHS for treatment of diseases caused by C-reactive protein. WO 2006/045319 describes the use of GHS in the treatment of renal and/or liver failure and complications thereof. More generally, WO 2005/097173 suggests the use of GHS for the treatment of ghrelin deficiency, including a wide array of therapeutic indications.
The myriad effects of ghrelin in humans have suggested the existence of subtypes for its receptor, although none have as yet been identified. (Torsello, A.; Locatelli, Y.; Melis, M. R.; Succu, S.; Spano, M. S.; Deghenghi, R.; Muller, E. E.; Argiolas, A.; Torsello, A.; Locatelli, V.; et al. Neuroendocrinology 2000, 72, 327-332.) However, a truncated, inactive form of GHS-R1a, termed GHS-R1b, was isolated and identified at the same time as the original characterization. Evidence is mounting that additional receptor subtypes could be present in different tissues to explain the diverse effects displayed by endogenous peptides and synthetic GHS. For instance, high affinity binding sites for ghrelin and des-acyl ghrelin have also been found in breast cancer cell lines, cardiomyocytes, and guinea pig heart that are involved in mediating the antiproliferative, cardioprotective and negative cardiac inotropic effects of the peptides. Similarly, specific GHS binding sites besides GHS-R1a and GHS-R1b have been found in prostate cancer cells. Further, ghrelin and des-acyl ghrelin exert different effects on cell proliferation in prostate carcinoma cell lines. (Cassoni, P.; Ghe, C.; Marrocco, T.; et al. E Eur. J. Endocrinol. 2004, 150, 173-184.) These various receptor subtypes may then be implicated independently in the wide array of biological activities displayed by the endogenous peptides and synthetic GHS. Indeed, recently, the existence of receptor subtypes was offered as an explanation for the promotion of fat accumulation by ghrelin, despite its potent stimulation of the lipolytic hormone, growth hormone. (Thompson, N. M.; Gill, D. A. S.; Davies, R.; Loveridge, N.; Houston, P. A.; Robinson, I. C. A. F.; Wells, T. Endocrinology 2004, 145, 234-242.) Further, this work suggested that the ratio of ghrelin and des-acyl ghrelin production could help regulate the balance between adipogenesis and lipolysis in response to nutritional status.
The successful creation of peptidic ghrelin analogues that separate the GH-modulating effects of ghrelin from the effects on weight gain and appetite provides strong evidence for the existence and physiological relevance of other receptor subtypes. (Halem, H. A.; Taylor, J. E.; Dong, J. Z.; Shen, Y.; Datta, R.; Abizaid, A.; Diano, S.; Horvath, T. L.; Culler, M. D. Neuroendocrinol. 2005, 81, 339-349; Halem, H. A.; Taylor, J. E.; Dong, J. Z.; Shen, Y.; Datta, R.; Abizaid, A.; Diano, S.; Horvath, T.; Zizzari, P.; Bluet-Pajot, M.-T.; Epelbaum, J.; Culler, M. D. Eur. J. Endocrinol. 2004, 151, S71-S75.) BIM-28163 functions as an antagonist at the GHS-R1a receptor and inhibits receptor activation by native ghrelin. However, this same molecule is a full agonist with respect to stimulating weight gain and food intake. Additionally, the existence of a still uncharacterized receptor subtype has been proposed based on binding studies in various tissues that showed differences between peptidic and non-peptidic GHS. (Ong, H.; Menicoll, N.; Escher, F.; Collu, R.; Deghenghi, R.; Locatelli, V.; Ghigo, E.; Muccioli, G.; Boghen, M.; Nilsson, M. Endocrinology 1998, 139, 432-435.) Differences between overall GHS-R expression and that of the GHS-R1a subtype in rat testis have been reported. (Barreiro, M. L.; Suominen, J. S.; Gaytan, F.; Pinilla, L.; Chopin, L. K.; Casanueva, F. F.; Dieguez, C.; Aguilar, E.; Toppari, J.; Tena-Sempere, M. Biol. Reproduction 2003, 68, 1631-1640.) A GHS-R subtype on cholinergic nerves is postulated as an explanation for the differential actions of ghrelin and a peptidic GHS on neural contractile response observed during binding studies at the motilin receptor. (Depoortere, I.; Thijs, T.; Thielemans, L.; Robberecht, P.; Peeters, T. L. J. Pharmacol. Exp. Ther. 2003, 305, 660-667.) Finally, WO 2006/009645 and WO 2006/009674 report the separation of the GI effects from the GH-release effects in animal models using macrocyclic ghrelin agonists, also suggesting that different subtypes are involved in these physiological effects.
The variety of activities associated with the ghrelin receptor could also be due to different agonists activating different signaling pathways as has been shown for ghrelin and adenosine, both of which interact as agonists at GHS-R1a. (Carreira, M. C.; Camina, J. P.; Smith, R. G.; Casanueva, F. F. Neuroendocrinology 2004, 79, 13-25.)
The functional activity of a GPCR has been shown to often require the formation of dimers or other multimeric complexes with itself or other proteins. (Prinster, S. C.; Hague, C.; Hall, R. A. Pharmacol. Rev. 2005, 57, 289-298; Park, P. S.; Filipek, S.; Wells, J. W.; Palczewski, K. Biochemistry 2004, 43, 15643-15656; Rios, C. D.; Jordan, B. A.; Gomes, I.; Devi, L. A. G-protein-coupled receptor dimerization: modulation of receptor function. Pharmacol. Ther. 2001, 92, 71-87; Devi, L. A. Trends Pharmacol. Sci. 2001, 22, 532-537.) Likewise, the activity of the ghrelin receptor might also be at least partially governed by such complexes. For example, certain reports indicate that interaction of GHS-R1a with GHRH (Cunha, S. R.; Mayo, K. E. Endocrinology 2002, 143, 4570-4582; Malagón, M. M.; Luque, R. M.; Ruiz-Guerrero, E.; Rodríguez-Pacheco, F.; García-Navarro, S.; Casanueva, F. F.; Gracia-Navarro, F.; Castaño, J. P. Endocrinology 2003, 144, 5372-5380) or between receptor subtypes (Chan, C. B.; Cheng, C. H. K. Mol. Cell. Endocrinol. 2004, 214, 81-95) may be involved in modulating the function of the receptor.
Further, the appetite regulating effects of ghrelin have been attributed to the constitutive activity of the receptor. (Holst, B. Schwartz, T. J. Clin. Invest. 2006, 116, 637-641; Holst, B.; Schwartz, T. W. Trends Pharmacol. Sci. 2004, 25, 113-117; Holst, B.; Holliday, N. D.; Bach, A.; Elling, C. E.; Cox, H. M.; Schwartz, T. W. J. Biol. Chem. 2004, 279, 53806-53817; Holst, B.; Cygankiewicz, A.; Jensen, T. H.; Ankersen, M.; Schwartz, T. W. Mol. Endocrinol. 2003, 17, 2201-221.) The recent observation that humans possessing a mutation in the ghrelin receptor that impairs constitutive activity are of short stature suggests the importance of the constitutive activity to the normal in vivo function of this receptor. (Pantel, J.; Legendre, M. Cabrol, S.; et al. J. Clin. Invest. 2006, 116, 760-768.)
The vast majority of reported approaches to exploiting the ghrelin receptor for therapeutic purposes have focused on modulating metabolic functions. Similarly, the vast majority of literature on GHS focuses on conditions that can be treated via its GH promoting actions. Some embodiments of the invention described herein, in particular, take advantage of selective activation of the ghrelin receptor to provide an avenue for the treatment of diseases characterized by GI dysmotility. The improved GI motility observed with ghrelin demonstrates that ghrelin agonists may be useful in correcting conditions associated with reduced or restricted motility. (Murray, C. D. R.; Kamm, M. A.; Bloom, S. R.; Emmanuel, A. V. Gastroenterology 2003, 125, 1492-1502; Fujino, K.; Inui, A.; Asakawa, A.; Kihara, N.; Fujimura, M.; Fujimiya, M. J. Physiol. 2003, 550, 227-240; Edholm, T.; Levin, F.; Hellström, P. M.; Schmidt, P. T. Regul. Pept. 2004, 121, 25-30; Locatelli, V.; Bresciani, E.; Bulgarelli, I.; Rapetti, D.; Torsello, A.; Rindi, G.; Sibilia, V. Netti, C. J. Endocrinol. Invest. 2005, 28, 843-848; Peeters, T. L. Gut 2005, 54, 1638-1649; Fruhwald, S.; Holzer, P.; Metzler, H. Wien. Klin. Wochenschr. 2008, 120, 6-17.)
Included among these conditions is post-operative ileus. (POI, Luckey, A.; Livingston, E.; Taché, Y. Arch. Surg. 2003, 138, 206-214; Baig, M. K.; Wexner, S. D. Dis. Colon Rectum 2004, 47, 516-526; Greewood-Van Meerveld, B. Exp. Opin. Emerging Drugs 2007, 12, 619-627; Senagore, A. J. Am. J. Health Syst. Pharm. 2007, 64, S3-S7; Maron, D. J.; Fry, R. D. Am. J. Ther. 2008, 15, 59-65.) POI is defined as the impairment of GI motility that routinely occurs following abdominal, intestinal, gynecological and pelvic surgeries. In the U.S. alone, 2.1 million surgeries annually induce POI, accounting for an economic impact of over $1 billion. POI is considered a deleterious response to surgical manipulation with a variable duration that generally persists for 72 hours. It is characterized by pain, abdominal distention or bloating, nausea and vomiting, accumulation of gas and fluids in the bowel, and delayed passage of stool. Patients are neither able to tolerate oral feeding nor to have bowel movements until gut function returns. POI leads to numerous undesirable consequences, including increased patient morbidity, the costly prolongation of hospital stays and, further, is a major cause of hospital readmission. In addition, opiate drugs given as analgesics after surgery exacerbate this condition due to their well-recognized side effect of inhibiting bowel function.
Surgical manipulation of the stomach or intestine causes a disorganization of the gut-brain signaling pathways, impairing GI activity and triggering POI. Ghrelin acts locally in the stomach to stimulate and coordinate the firing of vagal afferent neurons and thereby modulate gut motility. Thus, ghrelin accelerates gastric emptying in humans (Peeters, T. L. Curr. Opin. Pharmacol. 2006, 6, 553-558; Tack, J.; Depoortere, I.; Bisschops, R.; Delporte, C.; Coulie, B.; Meulemans, A.; Janssens, J.; Peeters, T. Gut 2006, 55, 327-333; Inui, A.; Asakawa, A.; Bowers, C. Y.; Mantovani, G.; Laviano, A.; Meguid, M. M.; Fujimiya, M. FASEB J 2004, 18, 439-456; Peeters, T. L. J. Physiol. Pharmacol. 2003, 54(Supp. 4), 95-103.) and is a potent agent proven to treat POI in animal models. (Trudel, L.; Tomasetto, C.; Rio, M. C.; Bouin, M.; Plourde, V.; Eberling, P.; Poitras, P. Am. J. Physiol. 2002, 282, G948-G952; Trudel, L.; Bouin, M.; Tomasetto, C.; Eberling, P.; St-Pierre, S.; Bannon, P.; L'Heureux, M. C.; Poitras, P. Peptides 2003, 24, 531-534; De Winter, B. Y.; De Man, J. G.; Seerden, T. C.; Depoortere, I.; Herman, A. G.; Peeters, T. L.; Pelckmans, P. A. Neurogastroenterol. Motil. 2004, 16, 439-446.) Ghrelin agonists duplicate the effects of ghrelin, thus targeting directly the underlying cause of POI to accelerate normalization of gut function and enable more rapid discharge from the hospital. (Kitazawa, T.; De Smet, B.; Verbeke, K.; Depoortere, I.; Peeters, T. L. Gut 2005, 54, 1078-1084; Poitras, P.; Polyino, W. J.; Rocheleau, B. Peptides 2005, 26, 1598-1601.) The reported anti-inflammatory actions of ghrelin may also play a role in ameliorating this condition. (Granado, M.; Priego, T.; Martin, A. I.; Villanua, M. A.; Lopez-Calderon, A. Am. J. Physiol. Endocrinol. Metab. 2005, 288, E486-E492; Iseri, S. O.; Sener, G.; Yuksel, M.; Contuk, G.; Cetinel, S.; Gedik, N.; Yegen, B. C. J. Endocrinol. 2005, 187, 399-406.)
Intravenous administration is often the preferred route of treatment for POI due to the impaired GI motility in these patients that impedes oral therapy. No agent is currently approved by the U.S. FDA specifically for the treatment of POI.
Another major motility disorder is gastroparesis, a particular problem for both type I and type II diabetics. (Camilleri, M. Advances in diabetic gastroparesis. Rev. Gastroenterol. Disord. 2002, 2, 47-56; Abell, T. L.; Bernstein, R. K.; Cutts, T. Neurogastrenterol. Motil. 2006, 18, 263-283; Camilleri, M. New Eng. J. Med. 2007, 356, 820-829.) Gastroparesis (“stomach paralysis”) is a syndrome characterized by delayed gastric emptying in the absence of any mechanical obstruction. It is variably characterized by abdominal pain, nausea, vomiting, weight loss, anorexia, early satiety, malnutrition, dehydration, gastroesophageal reflux, cramping and bloating. This chronic condition can lead to frequent hospitalization, increased disability and decreased quality of life. (Wang, Y. R.; Fisher, R. S.; Parkman, H. P. Am. J. Gastro. 2007, 102, 1-10.) Severe, symptomatic gastroparesis is common in individuals suffering from diabetes, affecting from 5-10% of diabetics for a total patient population of 1 million in the U.S. alone. Neuropathy is a frequent, debilitating complication of diabetes. Visceral neuropathy results in GI dysfunction, especially involving the stomach, and leading to impaired gastric motility. Ghrelin promotes gastric emptying both by stimulating the vagus nerve and via direct prokinetic action at the gastric mucosa. Moreover, recent clinical studies indicate that intravenous administration of the natural ghrelin peptide is an effective acute therapy in diabetic gastroparesis patients. (Binn, M.; Albert, C.; Gougeon, A.; Maerki, H.; Coulie, B.; Lemoyne, M.; Rabasa Lhoret, R.; Tomasetto, C.; Poitras, P. Peptides 2006, 27, 1603-1606; Murray, C. D. R.; Martin, N. M.; Patterson, M.; Taylor, S.; Ghatei, M. A.; Karm, M. A.; Johnston, C.; Bloom, S. R.; Emmanuel, A. V. Gut 2005, 54, 1693-1698; Tack, J.; Depoortere, I.; Bisschops, R.; Verbeke, K.; Janssens, J.; Peeters, T. Aliment. Pharmacol. Ther. 2005, 22, 847-853.)
A ghrelin agonist would therefore be highly effective in overcoming the fundamental motility barrier faced by gastroparesis patients and correcting this condition. As with POI, no accepted or efficacious therapy for diabetic gastroparesis is available and most current therapies aim to provide only symptomatic relief. Further, many of the therapeutics in development have a mechanism of action similar to earlier products that have failed in this indication. Surgical procedures may ameliorate the disease process, but offer no possibility of cure.
Post-surgical gastroparesis syndrome is a complication resulting from surgery characterized by delayed gastric emptying, postprandial nausea and vomiting, and abdominal pain. (Eckhauser, F. E., et al. Am. Surg. 1998, 64, 711-717; Tanaka, M. Surg. Today 2005, 35, 345-350.) These surgeries include gastrectomy, pancreatoduodenectomy, gastrojejunostomy in patients with pancreatic cancer and gastric surgery, as well as in patients with liver cirrhosis. (Doberneck, R. C.; Berndt, G. A. Arch. Surg. 1987, 122, 827-829; Bar-Natan, M.; Larson, G. M.; Stephens, G.; Massey, T. Am. J. Surg. 1996, 172, 24-28; Cohen, A. M.; Ottinger, L. W. Ann. Surg 1976, 184, 689-696; Isobe, H.; Sakai, H.; Satoh, M.; Sakamoto, S.; Nawata, H. Dig. Dis. Sci. 1994, 39, 983-987.) The only reported pharmaceutical agents shown to be useful for this syndrome are cisapride and erythromycin. (Takeda, T.; Yoshida, J.; Tanaka, M.; Matsunaga, H.; Yamaguchi, K.; Chijiiwa, K. Ann. Surg. 1999, 229, 223-229; Heidenreich, A.; Wille, S.; Hofmann, R. J. Urology 2000, 163, 545.) However, cisapride was removed from the market due, at least in part, to the appearance of life-threatening cardiac arythmia side effects. Further, erythromycin is not a desirable treatment due to the antibiotic activity potentially giving rise to resistance should it be used for non-infective purposes.
Opioid-induced bowel dysfunction (OBD, Kurz, A.; Sessler, D. J. Drugs 2003, 63, 649-671.) is the term applied to the confluence of symptoms involving the reduced GI motility that results from treatment with opioid analgesics. Approximately 40-50% of patients taking opioids for pain control experience OBD. It is characterized by hard, dry stools, straining, incomplete evacuation, bloating, abdominal distension and increased gastric reflux. In addition to the obvious short-term distress, this condition leads to physical and psychological deterioration in patients undergoing long-term opioid treatment. Further, the dysfunction can be so severe as to become a dose-limiting adverse effect that actually prevents adequate pain control. As with POI, a ghrelin agonist can be expected to counteract the dysmotility resulting from opioid use.
Two less common syndromes may also be helped through the GI motility stimulation effects of ghrelin and ghrelin agonists. Short bowel syndrome is a condition that occurs after resection of a substantial portion of small intestine and is characterized by malnutrition. Patients are observed to have decreased ghrelin levels resulting from loss of the ghrelin-producing neuroendocrine cells of the intestine. It is possible the short bowel feeds back on the release of the hormone. (Krsek, M.; Rosicka, M.; Haluzik, M.; et al. Endocr. Res. 2002, 28, 27-33.) Chronic intestinal pseudo-obstruction is a syndrome defined by the presence of chronic intestinal dilation and dysmotility in the absence of mechanical obstruction or inflammation. Both genetic and acquired causes are known to result in this disorder, which affects high numbers of individuals worldwide annually. (Hirano, I.; Pandolfino, J. Dig. Dis. 2000, 18, 83-92.)
Other conditions and disorders that could be addressed through stimulation of the ghrelin receptor are: constipation such as associated with the hypomotility phase of irritable bowel syndrome (IBS), delayed gastric emptying associated with wasting conditions, gastroesophageal reflux disease (GERD), gastric ulcers (Sibilia, V.; Muccioli, G.; Deghenghi, R.; Pagani, F.; DeLuca, V.; Rapetti, D.; Locatelli, V.; Netti, C. J. Neuroendocrinol. 2006, 18, 122-128; Sibilia, V.; Rindi, G.; Pagani, F.; Rapetti, D.; Locatelli, V.; Torsello, A.; Campanini, N.; Degenghi, R.; Netti, C. Endocrinology 2003, 144, 353-359.) and Crohn's disease. Ghrelin and ghrelin agonists also have been described as treatments for nausea, emesis or symptoms thereof. (U.S. Pat. Appl. Pub. No. 2005/277677; Rudd, J. A.; Ngan, M. P.; Wai, M. K.; King, A. G.; Witherington, J.; Andrews, P. L. R.; Sanger, G. J. Neurosci. Lett. 2006, 392, 79-83.)
Additionally, GI dysmotility is a significant problem in other mammals as well. For example, the motility dysfunction termed ileus or colic is the number one cause of mortality among horses. Further, ileus is one of the most common complications of equine intestinal surgery, in other words, post-operative ileus. This condition may also have a non-surgical etiology. Some horses may be predisposed to ileus based upon the anatomy and functioning of their digestive tract. Virtually any horse is susceptible to colic with only minor differences based upon age, sex and breed. Additionally, ileus may affect other animals, for example canines. (Roussel, A. J., Jr.; Cohen, N. D.; Hooper, R. N.; Rakestraw, P. C. J. Am Vet. Med. Assoc. 2001, 219, 72-78; Van Hoogmoed, L. M.; Nieto, J. E.; Snyder, J. R.; Harmon, F. A. Vet. Surg. 2004, 33, 279-285.)
Drug-induced adverse reactions are a well-known complication of all types of pharmacotherapy. Gastrointestinal side effects are among the most common complication experienced with pharmaceuticals, appearing in 20-40% of all cases. (Lewis, J. H. Am. J. Gastroenterol. 1986, 81, 819-834; Henry, D. A.; Ostapowicz, G.; Robertson, J. Clin. Gastroenterol. 1994, 8, 271-300.) Most seriously, an estimated 25% of drug-induced reactions in hospitalized patients involve the GI tract with potentially fatal outcomes in a small percentage of cases. (Stewart, R. B.; Cluff, L. E. Am. J. Dig. Dis. 1974, 19, 1-7.) Side effects can affect all portions of the GI tract. (Gore, R. M.; Levine, M. S.; Ghahremani, G. G. Abdom. Imaging 1999, 24, 9-16; Neitlich, J. D.; Burrell, M. I. Abdom. Imaging 1999, 24, 17-22; Neitlich, J. D.; Burrell, M. I. Abdom. Imaging 1999, 24, 23-38.) Such side effects can often only be addressed through reducing doses, thus often decreasing the efficacy of the medication. Additionally, patients often simply stop taking their medicines due to experiencing these side effects.
GI side effects are common in many established pharmaceutical classes, including anti-cholinergic agents (e.g. atropine, benzotropine, hyoscine, propantheline, scopolamine, trihexyphenidyl), tricyclic antidepressants (e.g. phenothiazines, amitriptyline, nortryptyline), monoamine uptake blocker antidepressants (e.g. desipramine, fluoxetine, citalopram, nomifensine), other psychoactive medications, cancer chemotherapy agents (e.g. vincristine), adrenergic agonists for hypertension, particularly β-agonists and α2-agonists, (e.g. isoproterenol, salbutamol, lidamidine, clonidine), dopaminergic agents (e.g. levodopa, bromocriptine, apomorphine), antimalarials (e.g. chloroquine, mepacrine), antispasmodic (e.g. pavatrine) and many other agents (e.g. zonisamide, pergolide, ibudilast, mexiletine, acarbose, sodium valproate, hexamethonium, alendronate).
In addition, many newer medications, although promising improved therapies for a range of diseases, are also subject to GI side effects. Among these are agonists of the glucagon-like peptide 1 (GLP-1), amylin and peptide YY (PYY) receptors that are very useful for treatment of diabetes and/or other metabolic disorders. Other pharmaceutical classes that exhibit GI side effects are proteasome inhibitors, a new chemotherapeutic, often as an adjunct therapy, for cancer, leukotriene receptor antagonists for asthma and other inflammatory diseases (e.g. pranlukast, Garcia, M.; Nakabayashi, T.; Mochiki, E.; Naga, N.; Pacheco, I.; Suzuki, T.; Kuwano, H. Dig. Dis. Sci. 2004, 49, 1228-1235), phosphodiesterase-5 (PDE-5) inhibitors (e.g. sildenafil: Dishy, V.; Pour, M. C.; Feldman, L. Clin. Pharm. Ther. 2004, 76, 281-286), and nicotinic acetylcholine receptors modulators (Mandl, P.; Kiss, J. P. Brain Res. Bull. 2007, 72, 194-200).
A number of new pharmaceutical treatments for metabolic disorders have been introduced or are in development. Unfortunately, many of these exhibit gastrointestinal (GI) side effects which can result in reduced efficacy, poor patient compliance and even removal of patients from medication.
For example, GLP-1 agonists such as exenatide are among the most effective new agents for treatment of diabetes. However, this mechanism of action also results in a significant reduction in gastric emptying. (Nauck, M. A.; Niedereichholz, U.; Ettler, R.; Holst, J. J.; Orskov, C.; Ritzel, R.; Schmiegel, W. H. Am. J. Physiol. 1997, 273, E981-E988; Tolessa, T.; Gutniak, M.; Holst, J. J.; Efendic, S.; Hellström, P. M. J. Clin. Invest. 1998, 102, 764-774; Little, T. J.; Pilichiewicz, A. N.; Russo, A.; Phillips, L.; Jones, K. L.; Nauck, M. A.; Wishart, J.; Horowitz, M.; Feinle-Bisset, C. J. Clin. Endocrinol. Metab. 2006, 91, 1916-1923; Barnett, A. Exp. Opin. Pharmacother. 2007, 8, 2593-2608.) Since delayed gastric emptying, or gastroparesis, is already a well-established problem for diabetic patients, this side effect exacerbates an already difficult situation.
Analogously, pramlintide has been introduced as an amylin agonist that is also useful for the treatment of diabetes. Unfortunately, inherent in its mechanism of action is reduced gastric emptying. (Young, A. Adv. Pharmacol. 2005, 52, 99-121.)
Peptide YY agonists likewise have potential utility for the treatment of metabolic disorders, but also reduce gastric emptying. (Chelikani, P. K.; Haver, A. C.; Reidelberger, R. D: Am. J. Physiol. 2004, 287, R1064-R1070.)
Similarly, proteasome inhibitors have been introduced as a useful therapy, either alone or in combination with other chemotherapeutic agents for treatment of a wide variety of hyperproliferative disorders, including many different types of cancers. However, one of these drugs, bortezomib, also results in delayed GI transit. (Perfetti, V.; Palladini, G.; Brunetti, L.; Sgarella, A.; Brugnatelli, S.; Gobbi P. G.; Corazza, G. R. Eur. J. Gastroenterol. Hepatol. 2007, 19, 599-601.)
Ghrelin agonists (as growth hormone secretagogues, GHS), but not those described in the present invention, have been employed in combination with a variety of other therapeutic agents, although not specifically to counteract drug-induced GI side effects. GHS in combination with selective estrogen receptor modulators (SERM) have been reported for treatment of muscoskeletal frailty (WO 99/65486, WO 99/65488, GB 2324726). EP 1149583 discusses the use of GHS with corticotrophin releasing factor (CRF) antagonists as medicaments for osteoporosis and cardiovascular diseases such as congestive heart failure. GHS have been described in combination with antidepressants for improvement in quality of life (WO 01/089570).
A number of combinations of pharmaceutical agents with GHS have been discussed for treating Alzheimer's, including with phosphodiesterase-4 inhibitors (WO 2004/087157), with β-amyloid modifying agents (WO 2004/110443) and p38 kinase inhibitors (WO 2005/058308). U.S. Pat. No. 6,657,063 reports combinations of GHS and β3-adrenergic agonists for the treatment of type II diabetes. GHS have been used in combination with GH for cachexia, decreased appetite and to increase food intake (WO 2005/097173; WO 2005/097174). WO 2006/092106 describes the use of a representative GHS, GHRP-6, with epidermal growth factor (EGF) for autoimmune and CNS diseases.
Combinations of other agents have been described for a variety of GI disorders. These include acetylcholinesterase inhibitors with anti-cholinergics agents as a treatment for chronic intestinal pseudo-obstruction (US 2004/082644). WO 2006/005613 discloses dipeptidyl peptidase IV inhibitors, in combination with 5-HT3 and 5-HT4 modulators for GI disorders.
Reports of combinations of drugs specifically for GI motility disorders are known, including 5-HT3 agonists with a second compound to treat diseases characterized by hypomotility (WO 2007/005780). 5-HT3 antagonists and 5-HT4 agonists with a second agent are described in WO 01/041748 and US 2004/092511.
Proton pump inhibitors (PPI) have been reported in combination with prokinetic agents (WO 2005/065664) and with GI motility agents (WO 2004/105795). PPI also have been reported in combination with compounds which modify gastrointestinal motility as an approach to the treatment of gastroesophageal reflux disease (GERD, U.S. Pat. Appl. Publ. No. 2006/0241134). Norcisapride, a prokinetic agent, has been used in combination with PPI and H2-antagonists, such as berberine, (WO 00/051583; WO 00/051584).
There remains, however, a need for additional combinations, such as the pharmaceutical compositions of the present invention, which can address the drug-induced GI side effects from certain drugs as outlined previously.
Importantly, for most of the above conditions, no specific, approved therapeutics exist and most therapies simply address symptomatic relief. However, specific modulation of the ghrelin receptor provides an opportunity to directly target the site of pathophysiological disturbance to better treat the underlying condition and improve clinical outcome. Further, macrocyclic ghrelin agonists have been shown not to stimulate concurrent GH secretion in animal models. (Venkova, K.; Fraser, G.; Hoveyda, H. R.; Greenwood-Van Meerveld, B. Dig Dis. Sci. 2007, 52, 2241-2248.) This separation of the gastrointestinal and GH effects has not previously been reported for any modulators of the ghrelin receptor. However, as already mentioned, the existence of analogues that separate the appetite control and GH modulatory effects associated with ghrelin has been recently reported (Halem, H. A.; Taylor, J. E.; Dong, J. Z.; et al. Eur. J. Endocrinol. 2004, 151, S71-S75).
WO 01/00830 discusses short gastrointestinal peptides (SGIP) that secrete growth hormone and also promote GI motility, but these were not shown to be due to action at the ghrelin receptor. Similarly, WO 2007/041278 describes peptide analogues of ghrelin that stimulate GI motility. U.S. Pat. Nos. 6,548,501 and 6,852,722 discuss specific non-peptidic GHS compounds useful for stimulation of GI motility. Similarly, WO 2006/010629, WO 2006/020930 and WO 2006/023608 describe ghrelin agonists (growth hormone secretagogues) for use in GI disorders. Moreover, other endogenous factors are known to stimulate secretion of GH, but do not promote GI motility. Indeed, many actually inhibit this physiological function. Specific receptor agonists such as the compounds of the present invention have much better potential to be selective and effective therapeutic agents.
Intl. Pat. Appl. WO 2006/009645 and WO 2006/009674 describe the use of macrocyclic compounds as ghrelin modulators for use in the treatment of GI disorders. The activity of one of these compounds in a rat model of POI has been reported. (Venkova, K.; Fraser, G.; Hoveyda, H. R.; Greenwood-Van Meerveld, B. Dig. Dis. Sci. 2007, 52, 2241-2248.) These macrocyclic compounds are structurally distinct from other compounds that have been found to interact at the ghrelin receptor as agonists. For example, significant work was devoted to the development of potent and selective GHS with a number of small molecule derivatives now being known as has been recently summarized. (Carpino, P. Exp. Opin. Ther. Patents 2002, 12, 1599-1618.) Specific GHS are described in the following: Intl. Pat. Appl. Publ. Nos. WO 89/07110; WO 89/07111; WO 92/07578; WO 93/04081; WO 94/11012; WO 94/13696; WO 94/19367; WO 95/11029; WO 95/13069; WO 95/14666; WO 95/17422; WO 95/17423; WO 95/34311; WO 96/02530; WO 96/15148; WO 96/22996; WO 96/22997; WO 96/24580; WO 96/24587; WO 96/32943; WO 96/33189; WO 96/35713; WO 96/38471; WO 97/00894; WO 97/06803; WO 97/07117; WO 97/09060; WO 97/11697; WO 97/15191; WO 97/15573; WO 97/21730; WO 97/22004; WO 97/22367; WO 97/22620; WO 97/23508; WO 97/24369; WO 97/34604; WO 97/36873; WO 97/38709; WO 97/40023; WO 97/40071; WO 97/41878; WO 97/41879; WO 97/43278; WO 97/44042; WO 97/46252; WO 98/03473; WO 98/10653; WO 98/18815; WO 98/22124; WO 98/46569; WO 98/51687; WO 98/58947; WO 98/58948; WO 98/58949; WO 98/58950; WO 99/08697; WO 99/08699; WO 99/09991; WO 99/36431; WO 99/39730; WO 99/45029; WO 99/58501; WO 99/64456; WO 99/65486, WO 99/65488; WO 00/01726; WO 00/10975; WO 00/48623; WO 00/54729; WO 01/47558; WO 01/92292; WO 01/96300; WO 01/97831; WO 2004/021984; WO 2005/039625; WO 2005/046682; WO 2005/070884; WO 2006/044359; U.S. Pat. Nos. 3,239,345; 4,036,979; 4,411,890; 5,492,916; 5,494,919; 5,559,128; 5,663,171; 5,721,250; 5,721,251; 5,723,616; 5,726,319; 5,767,124; 5,798,337; 5,830,433; 5,919,777; 6,034,216; 6,548,501; 6,559,150; 6,576,686; 6,639,076; 6,686,359; 6,828,331; 6,861,409; 6,919,315; 7,034,050 and U.S. Pat. Appl. Nos. 2002/0168343; 2003/100494; 2003/130284; 2003/186844; 2005/187237; 2005/233981.
Despite this immense body of work, cyclic compounds have rarely been found to act at the ghrelin receptor. When they have, antagonist activity has been more prevalent. For example, the 14-amino acid compound, vapreotide, an SRIH-14 agonist and somatostatin mimetic, was demonstrated to be a ghrelin antagonist. (Deghenghi R, Papotti M, Ghigo E, et al. Endocrine 2001, 14, 29-33.) The binding and antagonist activities of analogues of cortistatin, a cyclic neuropeptide known to bind nonselectively to somatostatin receptors, to the growth hormone secretagogue receptor have been reported (Intl. Pat. Appl. WO 03/004518). (Deghenghi R, Broglio F, Papotti M, et al. Endocrine 2003, 22, 13-18; Sibilia, V.; Muccioli, G.; Deghenghi, R.; Pagani, F.; DeLuca, V.; Rapetti, D.; Locatelli, V.; Netti, C. J. Neuroendocrinol. 2006, 18, 122-128.) In particular, one of these analogues, EP-01492 (cortistatin-8) has been advanced into preclinical studies for the treatment of obesity as a ghrelin antagonist, although a recent study casts doubts on its effectiveness. (Prodam, F.; Benso, A.; Gramaglia, E. Neuropeptides 2008, 42, 89-93.) These compounds exhibit an IC50 of 24-33 nM. In addition, these cyclic compounds and their derivatives, plus their use with metal binding agents have been described for their ability to be useful for radiodiagnostic or radiotherapeutic use in the treatment of tumors and acromegaly.
Cyclic and linear analogues of growth hormone 177-191 have been studied as treatments for obesity (WO 99/12969), with one particular compound, AOD9604, having entered the clinic for this indication. A compound already studied that is most similar to the molecules of the present invention is the GHS, G-7203 (EC50=0.43 nM), the cyclic peptide analogue of the growth hormone releasing peptide, GHRP-2. (Elias, K. A.; Ingle, G. S.; Burnier, J. P.; Hammonds, G.; McDowell, R. S.; Rawson, T. E.; Somers, T. C.; Stanley, M. S.; Cronin, M. J. Endocrinol. 1995, 136, 5694-5699.) However, simplification of this cyclic derivative led to still potent, linear compounds, whereas, for compounds of the invention, linear analogues have been found to be devoid of ghrelin receptor activity.
The macrocyclic compounds of the invention have been shown to possess ghrelin modulating activity, and in particular embodiments, as agonists. Macrocyclic peptidomimetics have been previously described as modulators of the ghrelin receptor and their uses for the treatment of a variety of GI and metabolic disorders summarized (Intl. Pat. Appl. Publ. Nos. WO 2006/009645; 2006/009674; WO 2006/046977; 2006/137974 U.S. Pat. Appl. Publ. Nos. 2006/025566; 2007/0021331; U.S. patent application Ser. No. 11/774,185) One of these compounds has entered the clinic. (Lasseter, K. C.; Shaughnessy, L.; Cummings, D.; et al. J. Clin. Pharmacol. 2008, 48, 193-202).
Although binding potency and target affinity are factors in drug discovery and development, also important for development of viable pharmaceutical agents are optimization of pharmacokinetic (PK) and pharmacodynamic (PD) parameters. A focus area for research in the pharmaceutical industry has been to better understand the underlying factors which determine the suitability of molecules in this manner, often colloquially termed its “drug-likeness.” (Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J. Adv. Drug Delivery Rev. 1997, 23, 3-25; Muegge, I. Med. Res. Rev. 2003, 23, 302-321; Veber, D. F.; Johnson, S. R.; Cheng, H.-Y.; Smith, B. R.; Ward, K. W.; Kopple, K. D. J. Med. Chem. 2002, 45, 2615-2623.) For example, molecular weight, log P, membrane permeability, the number of hydrogen bond donors and acceptors, total polar surface area (TPSA), and the number of rotatable bonds have all been correlated with compounds that have been successful in drug development. Additionally, experimental measurements of plasma protein binding, interaction with cytochrome P450 enzymes, and pharmacokinetic parameters are employed in the pharmaceutical industry to select and advance new drug candidates.
However, these parameters have not been widely explored or reported within the macrocyclic structural class. This lack of information creates challenges in drug development for such molecules. The macrocyclic compounds of the present invention have been found to possess desirable pharmacological characteristics, while maintaining sufficient binding affinity and selectivity for the ghrelin receptor, as illustrated in the Examples presented herein. These combined characteristics make the compounds of the present invention generally more suitable than previously reported macrocycles for development as pharmaceutical agents, particularly for use as orally administered agents or for chronic uses.